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A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) [a] is an electrical safety device that interrupts an electrical circuit when the current passing through a conductor is not equal and opposite in both directions, therefore indicating leakage current to ground or current flowing to another powered conductor.
To eliminate this confusion, the IEC decided to apply the term residual current device (RCD) to differential-current-operated ELCBs. Residual current refers to any residue when comparing current in the outbound and return currents in the circuit. In a single phase circuit this is simply the live or phase current minus the neutral current.
Since the earth leakage is restricted, leakage protection devices can be set to less than 750 mA. By comparison, in a solidly earthed system, earth fault current can be as much as the available short-circuit current. The neutral earthing resistor is monitored to detect an interrupted ground connection and to shut off power if a fault is detected.
Residual-current circuit breaker with overcurrent protection — combines the functions of a RCD and a MCB in one package. In the United States and Canada, these are called GFCI breakers. Earth leakage circuit breaker (ELCB) — This detects current in the earth wire directly rather than detecting imbalance. They are no longer seen in new ...
Normal circuit currents flow only in the neutral, and the protective earth conductor bonds all equipment cases to earth to intercept any leakage current due to insulation failure. The neutral conductor is connected to earth at the building point of supply, but no common path to ground exists for circuit current and the protective conductor.
Earth fault protection also requires current transformers and senses an imbalance in a three-phase circuit. Normally the three phase currents are in balance, i.e. roughly equal in magnitude. If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance.
Until the 1950s, isolated ground domestic mains supplies tended to have no Residual-Current Device or Earth Leakage Circuit Breaker , and too high a ground impedance to blow a fuse if a live-to-earth fault occurred. This could leave metalwork in the house live.
Plugs are designed such that the connection to the protective earth conductor should be the first connection when plugged in. It should also be the last to be broken when the plug is removed. [1] A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor.